In typical magnetic disk drive systems, the separation of the writer 100 and the reader 102 on a read/write (R/W) head 104 is around 5 μm, as shown in FIG. 1. As a result, there is a significant variation of the read/write offset at the typical skew angle range of about −15° to +15°. This creates a continuous change of offset between reader 102 and writer 100 in the range of about −1.29 μm to +1.29 μm from the inner diameter (ID) to the outer diameter (OD) of the disk media.
During the writing process, the writer 100 has to be on track but the positioning error signal (PES) for servo has to be reproduced by the reader 102. Due to the continuous change of read/write offset, there are many areas where the offset of the reader 102 and the writer 100 is not an integer number of the data track width. In order to be “servo on” during the writing process, the reader 102 thus has to micro-jog away from the data track center. For a good servo performance during the writing process, the micro jog of the reader should be occurring in a linear region of the PES transfer curve.
In dedicated servo systems, a dedicated servo layer 200 is provided in the disk media 202, as illustrated in FIG. 2. With a larger spacing between the R/W head 204 and the dedicated servo layer 200 in the dedicated servo system, the servo track width in the dedicated servo layer 200 can be double that of the data track width in the recording layer 206, without increasing the nonlinear portion of the PES transfer curve. In the existing DC+/− servo pattern for dedicated servo systems, DC+ servo regions e.g. 300 alternate with DC− servo regions e.g. 302 as illustrated in FIG. 3. For the dedicated servo system, the DC+/− servo pattern has low media noise from the servo layer and has minimal interference to the data signal. It is considered the best choice of servo pattern in terms of the servo influence to data signal performance.
Having the width of the servo track 304 exactly twice the width of the data track 306 could provide an optimised scenario in which the micro jog would occur around the zero crossing point 308 of the PES transfer curve 310, i.e. in the linear region of the PES transfer curve 310. However, in practice the magnetic read width of the reader and the magnetic write width of the writer vary from R/W head to R/W head. Since the servo writing of servo tracks does not capture the widths variation of R/W heads, the scenario of operating at the zero crossing point 308 of the PES transfer curve 310 cannot be achieved, in practice. Instead, the reader servos at nonlinear regions of PES transfer curve 310 at many skew angles during the writing, and the servo performance will thus degrade when writing at certain locations.
In other words, the current dedicated DC+/− servo systems do not provide the full range of linear PES to support the full range of linear micro jog for variable read/write offset of R/W heads with different magnetic write width.
Embodiments of the present invention provide a magnetic disk, a method of track following on a magnetic disk, and a method of writing a servo pattern in a dedicated servo layer of a magnetic disk that seek to address at least one of the above problems.